It works that way on my Mk IV. But don't assume ;--) Try it, with both volumes turned down.
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Call the manufacturer.
I have run parallel SE + BAL systems in the past, coverging at the POWER AMP. (CDP and PHONO AMP each go se to one PREAMP and bal to a 2nd PREAMP, the first PRE goes se to my POWER AMP and the other PRE goes bal to the same POWER AMP.) I always checked in first. Some said "no sweat", others said "no way". IIRC, it's a dicier move on outputs than on inputs, but I'd spend the dime on a call to be safe.
It all depends on how those inputs are arranged inside the amp. IF they are connected in some way all the time (ground? or negative leg?) then the power from the one that is on can go back up the line some to the one that is off and possibley cause problems. Transistors don't like current coming in backwards. (simplistic concept but just the idea
So it can be risky (cause damage) to the other source output, even if that source is turned off.
Or if you can read schematics, it's very easy to see if they are "connected in some way" as E. puts it. In the older Gordon Gow Commemorative Edition (the first MC275 with BAL inputs) the inputs were connected. So are the BAL and UNBAL inputs of the new (mono) 50th Anniversary MC75's.
However, I KNOW, both from the schematics and from actual operation, that the MC275 Mk IV has separated inputs. I also KNOW that the Mk V has the same circuit as the Mk IV (except no level controls on the UNBAL inputs.) However, since I haven't actually seen the Mk V scehmatics, or used one, I'm not going to guess ;--)
It won't hurt anything to try. A friend with a pair of the new MC75's tried it, and simply got crosstalk, which is how we found out that ITS input circuits ARE connected.
I tried that with my Octave V70SE and with the SE everything sounded right but the balanced was distorted. I was connecting a Meridian G08 to the Octave. I was trying to compare se to xlr without having to touch the cables and having both cables connected to the same source didn't work well in my case.
A simple thing to try, pending an answer from McIntosh, is to connect just one of the inputs, and see what you hear when the switch is set to select the other input. Do that for both inputs, one at a time.
If you hear music that approaches normal volume, it would mean that there is some interconnection between the two signal paths (which I suspect is not the case, given that there is a switch).
If you hear music at a very low volume, it would mean that there is low level crosstalk that is bypassing the switch, for example due to coupling via stray capacitances. That would imply that connecting both inputs at once is a viable approach, in terms of sonics, only if the unused source is not supplying a signal.
Also, if both inputs are connected my instinct is that it would be preferable to avoid changing the switch position unless the amp is turned off. Otherwise you may get thumps through the speakers, or worse if signals are present at one or both inputs.